In recent years, small-size portable apparatuses such as cellular phones have been widely used. Such small-size portable apparatuses employ small-size rechargeable (secondary) batteries as power supply. In order to make batteries smaller in size and usable for as long a period as possible, attempts have been made to improve battery performance and make apparatuses consume less power. It is desirable to widen the usable voltage range of batteries as much as possible to reduce batteries in volume and make them usable for a longer period of time. Accordingly, a step-up/down DC-DC converter, which can supply constant voltage to a load even when battery voltage becomes lower than a voltage required by the load, is employed as a power supply circuit. Japanese Laid-Open Patent Application No. 2000-166223 discloses such a step-up/down DC-DC converter. The step-up/down DC-DC converter, which is not selective in supply voltage, has the merit of being able to support a variety of input power sources such as batteries and AC adapters.
FIG. 1 is a circuit diagram showing a conventional step-up/down DC-DC converter 100.
The step-up/down DC-DC converter 100 includes an input terminal IN to which an upstream voltage Vin is input, an output terminal OUT from which a predetermined downstream voltage Vout is output, a PWM control part 101, and a voltage step-up/down part 102.
A comparator CMP13 compares a division voltage Vx generated by dividing the downstream voltage Vout by resistors Ra and Rb with a reference voltage Vref. Then, the comparator CMP13 generates an error signal from the comparison result, and outputs the error signal to a step-up-side comparator CMP12 and a step-down-side comparator CMP11.
A current supplied from a current source i11 and an external clock signal CLKe are input to each of a level shift circuit 111 and a sawtooth wave generator circuit 112. A shift by the level shift circuit 111 and the peak value of a sawtooth wave generated by the sawtooth wave generator circuit 112 have relative dependency. That is, even if the current supplied from the current source i11 varies, sawtooth waves in the level shift circuit 111 and the sawtooth wave generator circuit 112 vary while maintaining their relationship of the parallel translation of the same waveform. Accordingly, the sawtooth waves are unaffected by variations in the current supplied from the current source i11.
The sawtooth wave generator circuit 112 provides a sawtooth wave to the step-down-side comparator CMP11 as a step-down-side sawtooth wave. The sawtooth wave is also provided to the level shift circuit 111. The level shift circuit 111 shifts up the provided sawtooth wave by a predetermined voltage, and provides the shifted-up sawtooth wave to the step-up-side comparator CMP12 as a step-up-side sawtooth wave. The step-down-side and step-up-side comparators CMP11 and CMP12 compare their respective input sawtooth waves with the input error signal. For instance, if the error signal falls within a step-down-mode shift voltage range in the step-down-side sawtooth wave, the step-down-side comparator CMP11 outputs to a DC-DC converter control circuit 113 a mode switch signal Ddn to cause a step-down circuit to operate.
If the error signal falls within a step-up-mode shift voltage range in the step-up-side sawtooth wave, the step-up-side comparator CMP12 outputs to the DC-DC converter control circuit 113 a mode switch signal Dup to cause a step-up circuit to operate.
If the error signal falls within both the step-down-mode and step-up-mode shift voltage ranges, the step-down-side and step-up-side comparators CMP11 and CMP12 output the mode switch signal Ddn and Dup, respectively, to the DC-DC converter control circuit 113.
The DC-DC converter control circuit 113 generates a switching signal in response to the input mode switch signal Ddn and/or the input mode switch signal Dup, and outputs the generated switching signal to a predriver 114. The predriver 114 outputs the switching signal to the voltage step-up/down part 102 so as to perform step-up/down control of the downstream voltage Vout.
However, the conventional step-up/down DC-DC converter 100 requires the external clock signal CLKe to generate a sawtooth wave, and employs the level shift circuit 111 to generate a sawtooth wave for stepping up voltage. Thus, the conventional step-up/down DC-DC converter 100 requires an additional clock circuit and the level shift circuit 111. As a result, the circuit is increased in scale and complicated, which causes an increase in circuit space and cost.